An Overview of Approach to Corrosion Issues in Fukushima Daiichi Nuclear Power Station

摘要

The earthquake of magnitude 9.0 occurred on March 11, 2011 and the following tsunami caused the loss of ability to cool fuel in the reactors and the spent fuel pools (SFPs) in the Fukushima Daiichi nuclear power station. Since seawater was temporarily injected into the reactors and the SFPs to cool the fuel as an emergency action, various corrosion issues emerged. Temperatures in the reactors of Unit 1 to 3 exceeded 100 °C just after the accident and then decreased gradually to below 50 °C. The salt concentration in reactor water was assumed to be high due to seawater boiling down. In terms of keeping the stability of fuel cooling and the integrity against aftershocks, mitigating corrosion of carbon steel became a primary issue for the reactors. So far, the following corrosion mitigation measures have been applied to the reactors; (a) decreasing temperature, and deoxidization and sterilization (b) by deaeration of feed water, (c) by nitrogen gas injection into the reactors, (d) by hydrazine injection to cooling water, and (e) desalination of cooling water. The temperatures of SFP water in Unit 1 to 4 were from 47 to 93 °C just after the accident and then decreased to below 40 °C. The water quality corresponded to diluted seawater containing up to 2000 ppm Cl~-. However, pH at Unit 3 showed strong alkaline due to fallen concrete rubble. Examples of corrosion risks were localized corrosion of stainless steel pool liners and galvanic corrosion and alkaline corrosion of fuel racks made of Aluminum. Besides (a), (b), (d), and (e) listed above, (f) pH control have been applied to the SFPs for corrosion mitigation. Although cooling water quality have been improved to be equivalent of tap water at less than 50 °C, some corrosion issues remain to be solved in terms of long-term integrity.